Method of drilling and operating a subsea well

ABSTRACT

A method of preparing and operating a subsea well ( 23 ), comprising the steps of: locating a drill-through subsea tree ( 24 ) on a subsea wellhead; sealingly connecting a high pressure drilling riser ( 22 ) between the tree and a drilling platform at the sea surface; mounting a blow out preventer ( 16 ) at the top of the riser ( 22 ); drilling the well through the drilling riser and the subsea tree; and establishing a production connection between the tree ( 24 ) and a production collection facility at the sea surface through a production riser ( 52 ) which is separate from the drilling riser ( 22 ).

[0001] This invention relates to a method of preparing and operating asubsea well, and to subsea well components for use in such a method. Theinvention is particularly intended for use in floating drilling andproduction system used in the recovery of offshore oil and gas reservesin deep water environments. ‘Deep water’ environments are usuallyconsidered as those where the operating depth is 800 metres or more.

[0002] A number of deep water reservoirs have been or are proposed to bedeveloped using floating vessels with vertically tensioned high pressuredrilling and production risers. This approach allows both the drillingBOP (blow out preventer) and production tree to be located on the vessel(often referred to as a ‘dry tree’ arrangement) providing access to theBOP and production tree reducing the duration of drilling and workoveroperations and cost. In this arrangement particular attention must bepaid to well control requirements since in the event of a riser failurethe well can be left in an unstable condition, resulting in anuncontrolled blow-out situation. To achieve acceptable reliability dualcasing risers are utilised to provide redundant pressure barriers.However, as the search for hydrocarbons extends to even greater waterdepths the complexity of these high pressure dual casing risers andparticularly their suspended weight becomes a significant cost driver.

[0003] According to the invention, there is provided a method ofpreparing and operating a subsea well, the method comprising the stepsof

[0004] locating a drill-through subsea tree on a subsea wellhead

[0005] sealingly connecting a high pressure drilling riser between thetree and a drilling platform at the sea surface

[0006] mounting a blow out preventer at the top of the riser

[0007] drilling the well through the drilling riser and the subsea tree;and

[0008] establishing a production connection between the tree and aproduction collection facility at the sea surface through a productionriser which is separate from the drilling riser.

[0009] Use of this method offers the ability to efficiently drill,produce and workover subsea wells in deep water by combining manifoldeddrill-through spool trees, a high pressure drilling riser with surfaceBOP and free standing offset production risers.

[0010] This can simplify the riser design, reduce the number of risersrequired and simplify the interface with a vessel or platform with theobjective of reducing cost and improving safety.

[0011] In the invention, production risers can be run through themoonpool of a moored drilling vessel and a smaller vessel can be used todo the final installation onto the riser foundation.

[0012] The pressure retaining drilling riser may comprise two concentricriser pipes, the inner of the pipes being a high pressure riser and theouter of the pipes being a low pressure riser. The low pressure risercan then be first connected between the tree and the drilling platformwith a low pressure blow out preventer mounted at the top of the riser.The well can then be drilled to a first depth, the low pressure blow outpreventer removed and the high pressure riser run into the low pressureriser. A high pressure blow out preventer is then mounted at the top ofthe high pressure riser, and the well is drilled to a second, greaterdepth.

[0013] A plurality of wells can be drilled adjacent to one another, andthe production outflows from adjacent wells can be comingled in amanifold at the seabed before being introduced to the productionconnection to the sea surface.

[0014] The drilling platform and the production collection facility canbe provided on separate platforms/vessels at the sea surface.

[0015] The invention also provides a drill-through subsea tree adaptedfor use in the method set forth above. In particular, the tree may havean outlet for connection to a production pipe, and means for providing adirect sealing connection to a pressure retaining riser so that drillingcan take place through the riser and through the subsea tree.

[0016] The invention includes a permanently moored floating drilling andproduction system for deep water comprising: a) manifolded drill throughsubsea trees located (directly) below the floating vessel; b) a highpressure drilling workover riser and c) a surface BOP configured withthe objective of reducing riser numbers and tension requirement andmaintaining vertical wellbore access.

[0017] The invention also includes a riser system for use in drillingand producing a deepwater well from a permanently moored floating vesselcomprising: a high pressure vertically tensioned marine riser systemwith surface BOP for drilling operations extending downwardly from thesurface vessel and connected and sealed to a drill through subsea treethat is attached to a subsea wellhead located substantially below thesurface vessel.

[0018] The invention also includes a riser system for use in drillingand producing a deepwater well from a permanently moored floating vesselcomprising dual string concentric pipe arrangement comprising: 1) anouter riser extending from the surface vessel connected and sealed to adrill through subsea tree that is connected onto a subsea well fordrilling an initial low pressure interval; 2) a retrievable inner riserextending from the surface downwardly to the subsea tree inside theouter riser and connected and sealed on the bore of the subsea tree orwellhead.

[0019] The inner string can be a casing.

[0020] On completion of a well, the riser system can be disconnectedfrom the drill through subsea tree, lifted slightly and moved acrossonto and connected and sealed to another drill through subsea tree fordrilling or intervention.

[0021] The invention also extends to a floating drilling and productionvessel with multiple drill through subsea trees located below the vesselwith drilling and workover conducted vertically using high pressurerisers with surface BOP.

[0022] This surface vessel can use free standing production risers totransfer commingled fluids from drill through subsea trees to thedrilling and production vessel.

[0023] Free standing offset risers can be used to transfer commingledfluids from subsea trees to an adjacent storage.

[0024] The offset risers can be initially connected to thedrilling/production facility for early production and subsequentlydisconnected and reconnected to an adjacent storage facility. The riserscan be installed through the moonpool or over the side of a permanentlymoored drilling and production vessel.

[0025] The production risers can be assembled using threadedconnections.

[0026] The invention also extends to the installation of manifoldstructures through the moonpool of a moored drilling and productionvessel where the manifold is initially run in a vertical orientation inorder to pass the moonpool and subsequently rotated horizontally afterlanding on the seabed or in midwater.

[0027] Still further, the invention extends to the installation of nearneutrally buoyant rigid flowline spools to which the manifold has beenassembled using threaded connections and initially run in the verticalorientation and subsequently rolled over to the horizontal orientationafter passing through the moonpool of the vessel.

[0028] Yet another feature of the invention is the use of a radialorientation key in the bore of a high pressure drilling riser to locateand align a tubing hanger landed in the bore of a drill through spooltree

[0029] In the event that a single string drilling riser is utilised ashear ram module may be used at the base of the riser to isolate thewell in the event of a riser failure. The shear ram module will connectto the subsea tree mandrel via a remote connector and will have at itstop end a mandrel onto which the drilling riser is connected. In theunlikely event that the shear ram is actuated to close in the well, thedrilling riser is then retrieved and repaired prior to reinstallation onthe shut-in well.

[0030] The invention thus relates to an offshore production system fordeep and ultra deep water developments that allows drilling, productionand workover of subsea wells.

[0031] There are four main elements used in the production system, ie

[0032] Moored floating production unit

[0033] Drill through subsea trees with compact manifold

[0034] High pressure drilling riser

[0035] Free standing offset production risers

[0036] The floating production unit may take a number of different formsincluding barge, ship, semi-submersible, TLP (tension leg platform) orSpar. However, in its simplest form it consists of a flat bottom bargeconstructed from either steel or concrete. The barge provides drilling,production, storage and accommodation facilities with the drillingfacilities being located near the centre of the vessel where vesselmotions are smallest. A drilling derrick is located directly above acentral moonpool that facilitates installation of a high pressuredrilling riser.

[0037] A vertically tensioned high pressure drilling riser is proposed,similar to that used on existing deepwater developments. The riser pipeis constructed from steel tubulars, connected by flanged couplings. Theriser is rated to the maximum reservoir pressure and a surface BOP isused to control the flow of drilling fluids and returns in and out ofthe well bore. The drilling riser may be either a single string or adual string concentric arrangement. If a single string riser is used ariser base shear ram module may also be used immediately above the tree.The BOP is located in the moonpool directly below the derrick.

[0038] During drilling operations control of the subsea tree and riserbase shear ram module is provided via a control umbilical run on theoutside diameter of the drilling riser. Following connection of thedrilling riser to the tree control of the tree functions is provided andthe production control is isolated.

[0039] Subsea trees and manifolds are located on the seabed below theproduction vessel. The trees are ‘spool’ or ‘drill through’ designallowing full bore access to the well onto which they are connected. Thetrees can be installed on to the subsea wellhead on the bottom end ofthe drilling riser with subsequent drilling activities conducteddirectly through the tree.

[0040] Well intervention and light workover operations can be completedthrough a small bore high pressure riser typically 8⅝ inch diameter. Thesingle string riser is run down and attached to the upper mandrel of thetree in the same way as the drilling riser. A similar surface BOP, butof smaller internal diameter is used at the surface. Such a lightweightriser system allows access into the production tubing. For additionalsafety an isolation valve may be included at the base of the risercapable of shearing wireline, slickline and coiled tube.

[0041] A number of trees (typically five) are arranged on each of fourmanifolds, which commingles the flow from each tree and directs the flowto an adjacent production riser base. A number of such tree andmanifolds may be used, typically providing a total of twenty subseatrees.

[0042] Each manifold is connected to an adjacent offset production riservia spools that provide production, annulus access and controlfunctions. The offset riser consists of near vertical steel pipesconnected by threaded couplings. The risers are vertically supported bynear surface aircans which maintain tension in the riser sufficient towithstand environmental and operational loads. At the top of the riser aflexible pipe jumper is used to connect between the riser and theproduction vessel. The production riser may be single string for serviceie. water injection or concentric dual string for production where theouter annulus may be used for insulation of gas injection/lift.

[0043] It will be apparent to a person skilled in this technology thatthis arrangement greatly reduces the number of production risersrequired for such a development from approximately twenty to five, sincethe wells are manifolded subsea. This reduces riser steel weight,tensioning requirements and wellbay size. Furthermore the arrangementfacilitates subsea wellbore isolation at the subsea wellhead improvingsafety, reliability of such a system and simplifies the wellbay andmoonpool layout. Most importantly these benefits are provided withoutthe loss of vertical wellbore access for drilling and workover and withthe ability to use a high pressure drilling riser and surface BOP.

[0044] The invention will now be described in more detail by way ofexample only with reference to the accompanying drawings in which:

[0045]FIG. 1 is a general view showing the method of the invention inuse;

[0046]FIG. 1a is a view similar to that of FIG. 1 but showing analternative arrangement;

[0047]FIG. 2 is a cross-section through a subsea tree and manifold foruse in the invention;

[0048]FIG. 3 is a plan view of a manifold;

[0049]FIG. 4 is a side view of the manifold of FIG. 3;

[0050]FIG. 5 shows a detail of the offset riser upper assembly; and

[0051]FIG. 6 shows a detail of the offset riser base arrangement

[0052] Drilling takes place from a vessel 10 which consists of a steelor concrete barge with a central moonpool 12. The vessel is permanentlyspread moored for the duration of the field life or alternatively may beturret moored. Typical dimension of the barge are 175 m long, 60 m widthand a depth of 15 m. It will however be clear to the skilled man thatthese dimensions may be varied according to the requirements of eachparticular development. The main function of the barge is to providedrilling and workover for subsea wells that are located directly belowthe vessel. However, the barge may also provide other functions such aspersonnel accommodation, process and storage.

[0053] Drilling and workover of a well 23 takes place through a highpressure riser 22. Production, ie the bringing of oil or gas form thewell 23 to the surface, takes place through an offset production riser52.

[0054] Drilling and workover is conducted through the central moonpool12 which is typically 10 m×15 m plan area, allowing installation ofmanifolds, trees, drilling riser and offset production risers. Thedrilling facilities consist of a conventional derrick 14 and mud andpipe handling facilities. The drilling facilities are modular and can beskidded onto the barge 10 during barge construction and possibly removedat the end of the drilling phase. The arrangement uses a surface BOP 16,which is located within the moonpool immediately below the drill floor.Sufficient vertical space is provided to accommodate stroking of the BOPin the worst anticipated storm condition without impact with the hullstructure.

[0055] Flexible jumpers 18 from the offset production risers 50,52 areconnected to the barge via porches 20 located on the side of the barge.The porches are located away from the drilling area to provide goodseparation between drilling and production facilities for safetyreasons.

[0056] The drilling riser 22 extends below the barge 10 to the wells onthe seabed and is rated to resist the maximum shutin pressure of thereservoir. Isolation is achieved by the use of a surface BOP. The risermay be either single string or dual string, depending on particularreservoir parameters. A single string riser could have a diameter ofapproximately 24 inches and a dual string (concentric) riser could havepipe dimensions typically 22 inches diameter for the outer pipe and 13⅜inch diameter for the internal liner.

[0057] The drilling riser 22 is run through the moonpool of theproduction vessel and is assembled from a series of riser joints (22 a,22 b, 22 c, . . . ) using mechanical connections. At the bottom end theriser pipe is attached to the upper mandrel of a production tree 24 viaa hydraulic collet connector. The tree will be described in more detailwith reference to FIG. 2.

[0058] A tapered stress joint is used between the tree connector and thefirst standard riser joint to accommodate local stresses resulting fromenvironmental loading and vessel offset. The tapered stress joint is apipe section with an increasing wall thickness along its length toresist bending loads. The taper joint may be manufactured from steel ortitanium if lower wellhead loads are required. The bore of the taperjoint incorporates an orientation pin or similar device to alloworientation of the tubing hanger and tubing hanger running tool. The pinis hydraulically extended into the bore of the taper joint and impingeson a helical profile provided on the tubing hanger running tool.

[0059] In use, the riser 22 is lowered through the moonpool 12 andconnected to the subsea tree 24 at the top of a well 23. The tree 24 ispreinstalled on a subsea manifold 26. After connection to the subseatree the drilling riser is tensioned within the moonpool of the bargeusing a hydro-pneumatic system. The uppermost joint of the riser stringis machined with a profile to accept the BOP 16 and a conventionaldiverter (not shown) is located below the drill floor.

[0060] In an alternative embodiment, the subsea tree can be installed onthe bottom of the drilling riser and can be lowered to the seabed withthe riser.

[0061] Where a dual string riser 22 is being used, after drilling thetop hole section, an internal smaller diameter casing is run inside theouter pipe to provide a high pressure liner through which the remainingbottom hole section is drilled. The liner is assembled from threadedcasing and is latched into a profile inside the bore of the subseawellhead 24. This requires the liner to be installed through the taperjoint and bore of the subsea tree 36. Once the liner is latched andsealed to the bore of the wellhead it is pretensioned at the surfaceagainst the outer 22 inch pipe using a bowl and slip assembly. A highpressure surface BOP is attached to the upper end of the liner fordrilling the higher pressure bottom hole section. Alternatively theinternal liner can be latched and sealed in the bore of the subsea tree.

[0062] Following drilling to total depth the well is completed byinstallation of production tubing and tubing hangers. This is achievedin the case of the dual string drilling riser by removal of the innerliner to provide full bore access through the drilling riser to allowpassage of the tubing hanger, which is landed in the body of the tree.

[0063] After drilling and completing on one well the drilling riser isdisconnected from the tree, lifted slightly and then jumped across tothe next well to be drilled or requiring intervention, without retrievalto the surface. The subsea tree 24 (FIG. 2) is a ‘drill through’ design.

[0064]FIG. 2 shows the manifold 26, a wellhead 30 and a subsea tree 24mounted on the wellhead. This Figure shows a dual string riser with anouter low pressure drilling riser 34 and an inner high pressure riser36. The high pressure riser has to be sealed to the wellhead, and FIG. 2shows two possible ways in which this sealing can be completed. On theright hand side of the centre axis, the riser 36 is shown sealed byseals 38 directly in a bore in the well head 30. On the left hand sideof the centreline, the riser is shown sealed indirectly to the wellheadby seals 40 in the tree 24. The tree is then itself sealed to thewellhead 30 by further seals at 42.

[0065] The well casings are hung from hangers 70 in the wellhead 30. Thetree 24 has a downwardly flared connector collar 72 which locates overthe top of the tree. The tree has production valves 74 and a productionchoke 76. At the upper end of the tree, there is a re-entry funnel 78 tofacilitate re-entry to the well.

[0066] On completion of the well the tubing hanger is landed in the boreof the tree spool with horizontal wing outlets. When the tubing hangeris not installed, full bore access is provided through the tree foraccess into the lower wellbore. This allows the high pressure liner 36to be run through the tree and landed and locked inside the wellhead(see 38 in FIG. 2).

[0067] The tree 24 will have a machined profile on the bore of thespool. This will be used to latch and seal the internal tieback sleeve(if a dual concentric design is used).

[0068] A template 32 (see FIG. 3) is used to determine the positions ofthe wells. The template is designed so that it can be installed throughthe moonpool 12 of the barge 10. Each template has locations 44 throughwhich wells will be drilled. In the example shown there are locationsfor up to five wells. The locations are arranged in a row such that thetemplate is long and thin and can pass through the moonpool vertically.The template incorporates temporary mudmats for stability prior todrilling and incorporates piping and valving for serving each welldrilled through the template, and a manifold 26 to which the piping isconnected so that the manifold can commingle production flow and,distribute control functions to individual trees located in thelocations 44.

[0069] An alternative arrangement is to locate the wells off thetemplate and connect them to the template using jumpers. This allows thesize of the template structure to be reduced.

[0070] An umbilical 66 (FIG. 1) can be used to control some of the treefunctions. The umbilical will be run down the outside diameter of thedrilling riser and will terminate at a stab plate 68 adjacent to thebase riser connector. The stab plate 68 mates with a similar stab plate70 on the tree.

[0071] The templates are lowered through the moonpool vertically ondrill pipe and rotated to the horizontal after passing the keel or onthe seabed. The manifold can be installed complete with jumper spools 50that, in use, connect the manifold to the base of the offset productionriser 52. These jumper spools may be 200-300 m long. The spools areassembled in the moonpool using threaded connections and are neutrallybuoyant in water due to being air filled and coated with a thick andlightweight thermal insulation material.

[0072] The manifolds are lowered and positioned on the seabed so thatthe end of the jumper spools connects with or lands close topreinstalled foundation piles 51 onto which the offset risers areattached. Multiple manifold units can be used, depending on the totalnumber of wells required. The manifolds are positioned on the seabed toallow good access from the barge, to protect the wellheads from droppedobjects and to allow the required distribution of offset risers 52around the perimeter of the barge.

[0073] The offset risers 52 consist of a pipe in pipe configuration. Thecentral pipe diameter is sized for the main flow path and the annulusbetween the central and outer pipe is filled with air and a vapour phasecorrosion inhibitor which, together with the buoyancy material aroundthe outer pipe, provides thermal insulation to the production pipe.Alternatively the annulus can be used for gas lift or gas injection andwill then be filled with pressurised hydrocarbon gas. Preheating the gasprior to injection into the riser provides an effective means of heatingthe central pipe to maintain product arrival temperatures.

[0074] The outside surface of the outer pipe 52 is coated with acorrosion protection material such as fusion-bonded epoxy or thermallysprayed aluminium. Buoyancy material is attached to the large diameterpipe, which is sized such that the pipe section is near neutrallybuoyant in water in the production mode.

[0075] The production riser 52 is offset from the production vessel, andis tensioned using aircans 54 connected to the top of the riser 52. Theaircans are attached to the riser by an articulation 56 that allows theaircan to rotate independently to the riser.

[0076] Beneath the articulation a gooseneck assembly 58 (FIG. 5) islocated to provide a fluid outlet flow path to jumpers 18. The jumpersconnect the riser 52 to the vessel 10 and are assembled from flexiblepipe manufactured from steel reinforced thermoplastic materials. Thejumpers are configured in free hanging catenaries and connect to porches20 located on the perimeter of the barge. Alternatively the jumpers canbe connected directly to an adjacent storage facility 80 and not to thedrilling barge. This is shown in FIG. 1a. A third option is to connectthe jumpers to the drilling barge 10 for early production and at a laterdate transfer the jumpers over to the adjacent storage facility 80 forthe remaining life of field.

[0077] Below the gooseneck is a spool 60 machined with an internalprofile used to suspend and pre-tension the internal pipes of the riser52 inside the outer carrier.

[0078] The spool interfaces with the gooseneck assembly providing flowpaths and communication with the gooseneck. The design of the spool issimilar to that used for wellhead tubing hangers wherein a hanger,complete with seals and lock down mechanisms is located within an outerwellhead or bowl.

[0079] At the base of the production riser 52 (FIG. 6), the riser isconnected to a mandrel profile fabricated onto the upper end of a pilethat may be drilled and grouted or jetted. A flowbend 62 with outboardhub 63 is incorporated at the bottom of the riser string. The hub 63allows connection of the jumper spool 50 (which itself connects to thesubsea trees 24) via a vertically installed spool 64.

[0080] The system described here allows subsea wells to be drilled andthen brought into production in an efficient and simple manner.

[0081] It is an advantage of the invention that the production riserscan be run through the moonpool of the moored drilling vessel and asmaller vessel can be used to do the final installation onto the riserfoundation.

1. A method of preparing and operating a subsea well, the methodcomprising the steps of locating a drill-through subsea tree on a subseawellhead sealingly connecting a high pressure drilling riser between thetree and a drilling platform at the sea surface mounting a blow outpreventer at the top of the riser drilling the well through the drillingriser and the subsea tree; and establishing a production connectionbetween the tree and a production collection facility at the sea surfacethrough a production riser which is separate from the drilling riser. 2.A method as claimed in claim 1, wherein the high pressure drilling riseris disconnected from the tree after drilling of the well.
 3. A method asclaimed in claim 1 or claim 2, wherein there are more production risersthan drilling risers.
 4. A method as claimed in claim 3, wherein thereis a single drilling riser.
 5. A method as claimed in any precedingclaim, wherein a plurality of wellheads are manifolded together, and asingle production riser is associated with each manifold.
 6. A method asclaimed in any preceding claim, wherein the drilling platform is afloating, moored drilling vessel and the production risers are connectedto the vessel at a part of that vessel spaced from the location of thedrilling riser.
 7. A method as claimed in any preceding claim, whereinthe high pressure drilling riser comprises two concentric riser pipes,the inner of the pipes being a high pressure riser and the outer of thepipes being a low pressure riser.
 8. A method as claimed in claim 7,wherein the low pressure riser is first connected between the tree andthe drilling platform with a low pressure blow out preventer mounted atthe top of the riser, the well is drilled to a first depth, the lowpressure blow out preventer is removed, the high pressure riser is runinto the low pressure riser, a high pressure blow out preventer ismounted at the top of the high pressure riser, and the well is drilledto a second, greater depth.
 9. A method as claimed in claim 1, whereinthe drilling platform and the production collection facility areprovided on separate platforms at the sea surface.
 10. A method asclaimed in claim 9, wherein the production risers are initiallyconnected to the drilling platform for early production and aresubsequently disconnected and reconnected to the production collectionplatform.
 11. A method as claimed in claim 5, wherein the manifoldstructures are installed through the moonpool of a moored, floatingvessel and are initially run in a vertical orientation in order to passthe moonpool and are subsequently rotated horizontally after landing onthe seabed or in midwater.
 12. A floating drilling and production systemfor deep water comprising a floating vessel, a drill-through subsea treelocated below the floating vessel, a high pressure drilling workoverriser extending between the tree and the vessel, a blow-out preventerlocated on the vessel at the top of the riser, and a production riserextending from the tree to near the water surface.
 13. A floatingdrilling and production system as claimed in claim 12, wherein there area plurality of subsea trees and the drilling riser has means by which itcan be selectively coupled to one of the trees and can be moved from onetree to another.
 14. A floating drilling and production system asclaimed in claim 12 or claim 13, wherein the drilling riser isvertically tensioned.
 15. A floating drilling and production system asclaimed in claim 14, wherein the drilling riser is tensioned usingaircans.
 16. A floating drilling and production system as claimed inclaim 13, wherein a plurality of wells are connected to a subseamanifold, and the production riser is connected to the manifold.
 17. Afloating drilling and production system as claimed in claim 16, whereinthere are a plurality of manifolds, and each manifold has a productionriser.
 18. A floating drilling and production system as claimed in anyone of claims 12 to 17, wherein the drilling riser comprises a dualstring concentric pipe arrangement with an outer riser extending fromthe floating vessel connected and sealed to a drill through subsea treeconnected onto a subsea well, for drilling an initial low pressureinterval, and a retrievable inner riser extending from the surface tothe subsea tree inside the outer riser and connected and sealed on thebore of the subsea tree or wellhead.
 19. A floating drilling andproduction system as claimed in claim 18, wherein the inner string is acasing.
 20. A floating drilling and production system as claimed in anyone of claims 12 to 19, wherein the production riser or risers areinstalled through the moonpool of the floating vessel.
 21. A floatingdrilling and production system as claimed in any one of claims 12 to 20,wherein the production risers are assembled using threaded connections.22. A floating drilling and production system as claimed in any one ofclaims 12 to 21, wherein a control umbilical is run on the outsidediameter of the drilling riser to provide control of the subsea tree.23. A floating drilling and production system as claimed in any one ofclaims 12 to 22, wherein each manifold is connected to an adjacentoffset production riser via spools that provide production, annulusaccess and control functions.
 24. A floating drilling and productionsystem as claimed in any one of claims 12 to 23, wherein the or eachoffset riser consists of near vertical steel pipes connected by threadedcouplings.
 25. A floating drilling and production system as claimed inany one of claims 12 to 24, wherein the or each production riser isvertically supported by near surface aircans which maintain tension inthe riser sufficient to withstand environmental and operational loads.26. A floating drilling and production system as claimed in claim 25,wherein a flexible pipe jumper is used at the top of the riser toconnect between the riser and the production vessel.
 27. A floatingdrilling and production system as claimed in any one of claims 12 to 26,wherein the production riser is single string.
 28. A floating drillingand production system as claimed in any one of claims 12 to 26, whereinthe production riser is concentric dual string for production and theouter annulus may be used for insulation of gas injection/lift.
 29. Afloating drilling and production system as claimed in any one of claims12 to 21, wherein the or each manifold has near neutrally buoyant rigidflowline spools.
 30. A floating drilling and production system asclaimed in any one of claims 12 to 21, wherein a radial orientation keyis provided in the bore of the high pressure drilling riser to locateand align a tubing hanger landed in the bore of a drill-through spooltree.
 31. A floating drilling and production system as claimed in anyone of claims 12 to 17, wherein the drilling riser is a single stringdrilling riser, and a shear ram module is provided at the base of theriser to isolate the well in the event of a riser failure.
 32. Afloating drilling and production system as claimed in claim 31, whereinthe shear ram module connects to the subsea tree mandrel via a remoteconnector and has at its top end a mandrel onto which the drilling riseris connected.